24.3 Solar Energy and Winds
About half of the
sunlight that
reaches Earth is
absorbed by the
surface. The rest
is either reflected
back into space or
absorbed in the
atmosphere.
20% of incoming sunlight
absorbed by clouds and gases
50%
5% reflected
absorbed by surface
by surface
25% reflected by
clouds, dust, and gases
24.3 Solar Energy and Winds
Energy in the Atmosphere
What happens to the energy Earth receives
from the sun?
Some solar energy that reaches Earth’s
atmosphere is reflected back, some is
absorbed by the atmosphere, and some is
absorbed by Earth’s surface.
24.3 Solar Energy and Winds
Energy in the Atmosphere
How is energy transferred within the
troposphere?
Energy is transferred within the troposphere
in three ways: radiation, convection, and
conduction.
24.3 Solar Energy and Winds
Energy in the Atmosphere
About 30 percent of the incoming solar
energy is reflected back into space by clouds,
dust in the air, gases, and Earth’s surface.
About 20 percent of the sun’s energy is
absorbed by clouds and gases.
About half of the solar energy passes through
the atmosphere and is absorbed by the
surface.
24.3 Solar Energy and Winds
Energy in the Atmosphere
The atmosphere is heated primarily by energy
that is reradiated by Earth’s surface.
• The energy radiated back into the atmosphere
is mostly infrared radiation.
• Gases in the atmosphere, including water vapor
and carbon dioxide, allow visible light to pass
through but absorb most infrared radiation.
24.3 Solar Energy and Winds
Energy in the Atmosphere
• These gases radiate some of this absorbed
energy back to Earth’s surface, warming the
lower atmosphere in a process called the
greenhouse effect.
• Without the greenhouse effect, Earth’s
surface would be much cooler than it is.
24.3 Solar Energy and Winds
Energy in the Atmosphere
When Earth’s
surface is
heated, much of
this energy is
radiated back as
infrared
radiation.
Solar
radiation
Infrared
radiation
24.3 Solar Energy and Winds
Energy in the Atmosphere
Energy is transferred within the troposphere.
• Radiation from the sun heats Earth’s surface,
which then radiates heat skyward.
• The air in direct contact with Earth’s surface is
heated by conduction.
• Warm air near the surface expands and rises,
and cooler, denser air sinks, forming convection
currents that move heat through the
troposphere.
24.3 Solar Energy and Winds
Energy in the Atmosphere
A. Radiation Much of the sun’s
radiation reaches Earth’s
surface, where it heats the land
and water. Land and water
radiate heat back into the
atmosphere.
B. Conduction The conduction
process transfers heat from
land and water directly to the
few meters of air nearest
Earth’s surface.
C. Convection Convection
moves heat through the
troposphere. As surface air is
heated by radiation and
conduction, rising warm air is
replaced by denser,
downward-flowing cool air.
A
C
B
24.3 Solar Energy and Winds
Wind
What causes winds?
Winds are caused by differences in air
pressure.
24.3 Solar Energy and Winds
Wind
Air naturally flows from areas of higher
pressure to areas of lower pressure.
• This flow is wind, which is the mainly horizontal
movement of air.
• Larger pressure differences produce stronger
winds.
24.3 Solar Energy and Winds
Wind
Differences in air pressure are often caused
by the unequal heating of Earth’s surface.
•
•
•
•
As air is heated, it expands.
As it becomes less dense, air rises.
Cooler, denser air flows in to replace it.
This process occurs on both local and global
scales, producing local and global winds.
24.3 Solar Energy and Winds
Local Winds
What are some examples of local winds and
global winds?
The breezes that occur where land meets a
large body of water are examples of local
winds.
24.3 Solar Energy and Winds
Local Winds
On a hot summer day, there is often a cool
breeze blowing in from the water to the
beach. This breeze is an example of a local
wind, a wind that blows over a short distance.
Local winds are caused by the unequal
heating of Earth’s surface within a small
region.
24.3 Solar Energy and Winds
Local Winds
Water has a higher specific heat than land
and takes longer to heat up and cool down.
• The sun heats the land more quickly than it
heats the water.
• The air above the land becomes warmer than
the air above the water.
• The warm air expands and rises. The cooler air
over the water flows toward the land, creating a
sea breeze.
24.3 Solar Energy and Winds
Local Winds
At night, these temperature and pressure
conditions are reversed.
• Land cools off more quickly than water.
• The cooler air over land has a higher density
than the warmer air over water.
• The result is a land breeze, where cooler air
over land moves toward water.
24.3 Solar Energy and Winds
Local Winds
Sea breezes and land breezes are local winds.
Sea breeze
Land breeze
Warm air rising
Cooler air
moving toward
the land
Warm air rising
Cooler air
moving toward
the water
24.3 Solar Energy and Winds
Global Winds
What are some examples of global winds?
Trade winds, westerlies, and polar easterlies
are examples of global winds.
24.3 Solar Energy and Winds
Global Winds
Convection Cells
Winds that blow over long distances from a
specific direction are called global winds.
• Global winds are caused by the unequal heating of
Earth’s surface across a large region.
• Global winds move in a series of huge bands called
convection cells.
24.3 Solar Energy and Winds
Global Winds
Bands of wind are caused by temperature
variations across Earth’s surface.
• At the equator, temperatures tend to be warmer
than at other latitudes.
• Warm air rises at the equator, creating a lowpressure region.
• This warm air is replaced by cooler air brought
by global winds.
• Higher in the atmosphere, air blows away from
the equator toward the poles.
24.3 Solar Energy and Winds
Global Winds
Earth is surrounded by a set of global wind belts.
Earth’s rotation
Dry air sinks over
the world’s deserts
Warm air rises at
the equator until it
reaches the top of
the troposphere.
The circulating air
patterns are called
“convection cells.”
Polar easterlies
The area where
the trade winds die
out is known as
the doldrums
Very cold air sinks at the
poles and flows outward,
creating winds called
polar easterlies.
24.3 Solar Energy and Winds
Global Winds
The trade winds are wind belts just north and
south of the equator.
In the Northern Hemisphere, they blow from
the northeast to the southwest.
24.3 Solar Energy and Winds
Global Winds
The prevailing westerlies occur between 30°
and 60° latitude in both hemispheres. These
winds generally blow from west to east over
much of North America.
The polar easterlies extend from 60° latitude
to the poles in both hemispheres.
24.3 Solar Energy and Winds
Global Winds
For hundreds of years,
sailing ships have relied
on global winds to
transport cargo across the
oceans.
24.3 Solar Energy and Winds
Global Winds
If Earth were not rotating on its axis, global
winds would move in roughly straight paths
from the poles to the equator.
The curving effect that Earth’s rotation has on
all free-moving objects, including global
winds, is called the Coriolis effect.
24.3 Solar Energy and Winds
Global Winds
A. A rocket launched
from the North Pole
toward the equator
would move in a
straight line if Earth
were not rotating.
B. The Coriolis effect
causes such a
rocket to appear to
curve to the right.
Non-rotating Earth
Movement of
rocket
Equator
Rotating Earth
Movement of
rocket
Equator
Direction of Earth’s rotation
24.3 Solar Energy and Winds
Global Winds
Monsoons
Seasonal changes in the heating of Earth’s
surface affect the circulation of the atmosphere.
• A monsoon is a wind system that is characterized by
seasonal reversal of direction.
• Monsoons are similar to land and sea breezes except
that they occur on a much wider scale and longer
time frame.
24.3 Solar Energy and Winds
Global Winds
Jet Stream
Global wind patterns are also affected by fastmoving streams of air at high altitudes.
• A belt of high-speed wind in the upper troposphere is
called a jet stream.
• Jet streams are caused by great differences in air
pressure that develop at high altitudes.
24.3 Solar Energy and Winds
Assessment Questions
1. How does most of the heating of the atmosphere
occur?
a.
b.
c.
d.
solar energy reflected by the atmosphere
solar energy absorbed by the atmosphere
solar energy reflected from the land and oceans
solar energy absorbed then radiated by the surface
24.3 Solar Energy and Winds
Assessment Questions
1. How does most of the heating of the atmosphere
occur?
a.
b.
c.
d.
solar energy reflected by the atmosphere
solar energy absorbed by the atmosphere
solar energy reflected from the land and oceans
solar energy absorbed then radiated by the surface
ANS: D
24.3 Solar Energy and Winds
Assessment Questions
2. Which of the following types of wind is a local
wind?
a.
b.
c.
d.
trade winds
jet stream
land breeze
monsoon
24.3 Solar Energy and Winds
Assessment Questions
2. Which of the following types of wind is a local
wind?
a.
b.
c.
d.
trade winds
jet stream
land breeze
monsoon
ANS:
C